1. Field of the Invention
The present invention relates to a twin clutch transmission, and more specifically to a twin clutch transmission capable of reducing the frequency of hit noises occurring in a dog clutch when shifting from neutral to a first speed.
2. Description of Background Art
A transmission is known that includes a plurality of pairs of gears between a main shaft and a countershaft wherein a speed change operation is performed by driving an axially, slidably supported sleeve or speed change gear by means of a shift fork.
JP-A No. 2005-214215 discloses the construction of a transmission wherein a shift fork is driven by a shift drum and wherein a synchronous engagement mechanism is provided for a sleeve that is driven by the shift fork. The synchronous engagement mechanism is a well-known mechanism disposed between a sleeve and a speed change gear that mesh with each other by sliding axially for allowing a smooth shift by synchronizing the rotations of both parts by sliding friction.
Normally, sequential transmissions using a shift drum do not employ the synchronous engagement mechanism as described above but often causes adjacent sleeves or change speed gears to engage with each other by means of a dog clutch comprising a plurality of dog teeth and dog recesses. In transmissions employing dog clutches, if there is a difference in the rotation speed between a main shaft and a countershaft, a hit noise may occur when engaging a dog clutch. Further, in a twin clutch transmission wherein its main shaft has a double structure composed of an inner main shaft and an outer main shaft and wherein a first clutch and a second clutch are provided that transmit rotation drive force of the inner main shaft and the outer main shaft respectively, it is possible to shift to an adjacent speed stage by switching the connection condition of a clutch. However, considering a hit noise occurring when engaging the dog clutch, there still is a room for improving the timing of engagement of a clutch at each shift gear.
FIG. 5 is a development view of a shift drum 100 of a conventional twin clutch transmission. SM1 and SM2 are guide grooves with which cylindrical convex portions 71b and 72b (shown by dotted lines) of a main shaft-side shift fork are to be engaged. SC1 and SC2 are guide grooves with which cylindrical convex portions 81b and 82b (shown by dotted lines) of a countershaft-side shift fork are to be engaged. When the rotational position of a shift drum 30 is at neutral (N), the cylindrical convex portions 81b and 82b are at the “C N-N” position shown on the left in the drawing while the cylindrical convex portions 71b and 72b are at the “M N-N” position shown on the right in drawing. Also, predetermined rotational positions following the “C N-N” shown on the left in the drawing and the predetermined rotational positions following the “M N-N” shown on the right in the drawing are set at 30 degree intervals, respectively.
At a neutral position, the cylindrical convex portions 81b, 82b, 71b, and 72b of each shift fork are at a middle, middle, right, and left positions respectively, and the dog clutch for each gear is not engaged. When the shift drum 30 is rotated up to the positions (“C 1-N” and “M 1-N”) corresponding to the first speed gear from the neutral position, the cylindrical convex portion 81b shifts from a middle position to a left position to engage the dog clutch for the first speed gear (thick arrows in the drawing). At this time, the axial positions of the other cylindrical convex portions 71b, 72b, and 82b have not changed.
This twin clutch transmission is so configured that, when the dog clutch for first the speed gear is engaged to complete a shift to first speed, the shift drum 30 is automatically rotated by 30 degrees in the shift-up direction in preparation for shifting to the second speed. This rotary operation is called “preliminary shift for shift-up” which is intended to be able to complete a shift simply by switching the connection condition of the twin clutch when a speed change order to second speed is issued. The preliminary shift for shift-up causes the shift forks to move to the “C 1-2” and “M 1-2” positions shown on the left and right in the drawing, respectively. Also, the preliminary shift for shift-up causes the SC2 to switch from a middle position to a right position so as to move the cylindrical convex portion 82b to the right position, thereby engaging the dog clutch for a second speed gear.
As described above, in conventional twin clutch transmissions, since the dog clutch for a first speed gear and the dog clutch for a second speed gear engage with different timings, there is a possibility that a hit noise occurs twice in the dog clutch when shifting from neutral to a first speed.